A laboratory-scale membrane bioreactor (MBR) with a gravity drain was tested for dyeing and printing wastewater treatment from a wool mill. The MBR was operated with continuous permeate by gravity and without chemical cleaning for 135 days. Results showed that excellent effluent quality could meet the reuse water standard in China. The average concentrations of COD, BOD5, turbidity and color in the effluent were 36.9 mg l(-1), 3.7 mg l(-1), 0.2 NTU and 21 dilution times (DT), respectively. The average removal rates of COD, BOD5, turbidity and color were 80.3%, 95.0%, 99.3% and 58.7%, respectively. The membrane flux increased with increasing of aeration intensity, and its increasing rate was related to pressure-heads. The higher the pressure-head, the greater the impact of aeration intensity on membrane flux. Statistical analysis also showed that both the pressure-head and aeration intensity significantly affected membrane flux. Due to its compact design, simple operation and easy maintenance, MBR with a gravitational filtration system hs low energy consumption and is cost-effective to build and operate. If the life expectancy of the membrane is set for 3-4 years and the membrane flux is set at 15 l/m(2.)h, such a MBR would be very competitive.